How soil has been made 125 



peel it off you can see that it has eaten away some of 

 the rock. When the lichen dies it may change into 

 food for other plants. 



We have learnt these things about soil formation. 

 First of all the rocks break up into fragments through 

 the splitting action of freezing water, the dissolving 

 action of liquid water, and other causes. This process 

 goes on till the fragments are very small like soil particles. 

 Then plants begin to grow, and as they die and decay 

 they give rise to the black humus that we have seen is 

 so valuable a part of the soil (p. 51). This is how very 

 many of our soils have been made. But the action of 

 water does not stop at breaking the rock up into soil ; it 

 goes further and carries the particles away to the lower 

 parts of the river bed, or to the estuary, to form a 

 delta, and mud flats that may be reclaimed, like Romney 

 Marsh in England and many parts of Holland have been. 

 Many of our present soils have been formed in this way. 

 Finally the particles may be carried right away to sea 

 and spread out on the bottom to lie there for many ages, 

 but they may become dry land again and once more be 

 soil 



One thing more we learnt from the river Stour. 

 Why did it flow quickly at the bridge and slowly else- 

 where ? We knew that the soil round the bridge was 

 gravelly, whilst up and down the stream it was clayey. 

 The river had not been able to make so wide or so deep 

 a bed through the gravel as it had through the clay, and 

 it could therefore be forded here. We knew also that 

 there was a gravel pit at the next village on the river, 

 where also there was a bridge and had been a ford, 

 and so we were able to make a rough map like Fig. 

 57, showing that fords had occurred at the gravel 



